(1) Field of the Invention
This invention relates to engine fuel control systems which incorporate an air fuel ratio sensor.
(2) Prior Art
Various fuel control systems are known in the prior art in which the quantity of fuel fed to the engine is controlled by sensors in the exhaust gas which give an indication of the air fuel ratio. Nevertheless, it remains extremely difficult to compensate for the ever changing operating conditions of the engine, the variations among different engines and so on as to always operate the engine with a predetermined air fuel ratio. This drawback may become critical when the engine is equipped with a catalytic converter for reducing undesirable components of the exhaust gases.
A widely used technique to control the air fuel ratio in stoichiometric feedback controlled fuel metering systems is limit cycle integral control. The word "feedback" is used in this context to indicate the use of an air fuel ratio detected in the exhaust gas to govern the air fuel ratio input to the engine. In this technique, there is a constant movement of a fuel metering component in a direction that always tends to counter the instantaneous air fuel ratio indication given by a typical two state exhaust gas oxygen (EGO) sensor. For example, every time an EGO sensor indicates a switch from a rich to a lean air fuel ratio mode of operation, the direction of motion of a typical carburetor's metering rod reverses to create a richer air fuel ratio condition until the sensor indicates a change from a lean to a rich air fuel ratio condition. Then, the direction of motion of the metering rod is reversed again this time to achieve a leaner air fuel ratio condition.
Referring to FIGS. 1a and 1b, step like changes in the sensor output voltage initiate ramp like changes in the actuator control voltage. When using the limit cycle integral control, the desired air fuel ratio can only be attained on an average basis since the actual air fuel ratio is made to fluctuate in a controlled manner about the average value. The limit cycle integral control system can be characterized as a two-state controller with the mode of operation being either rich or lean. The average deviation from the desired value is a strong function of a parameter called engine transport delay time, .tau.. This is defined as the time it takes for a change in air fuel ratio, implemented at the fuel metering mechanism, to be recognized at the EGO sensor, after the change has taken place.
The engine transport delay time is a function of the fuel metering system's design, engine speed, air flow, and EGO sensor characteristics. A typical time delay is about 5 to 10 engine revolutions. Because of this delay time, a control system using a limit cycle technique always varies the air fuel ratio about a mean value in a cyclical manner. For example, a richer fuel ratio is typically followed by a leaner air fuel ratio with air fuel ratio overshoots occurring during the transition. The shorter the transport delay time is, the higher will be the frequency of rich to lean and lean to rich air fuel ratio fluctuations and the smaller will be the amplitudes of the air fuel ratio overshoots.
Feedback control of the air fuel ratio is typically necessary to achieve operation in the "window" of a three way catalyst. The window is the chemical composition of the exhaust gas on which the catalyst can act most efficiently to reduce any undesirable exhaust. If a stoichiometry-only air fuel sensor, such as zirconium dioxide sensor, is used, the typical result is the above described limit cycle control in which the air fuel ratio perpetually oscillates about the desired mean control value. That is, the zirconium dioxide sensor just provides an indication of the exhaust air fuel ratio with respect to stoichiometry by having a steep transition in amplitude at the stoichiometric air fuel ratio. However, if a titanium dioxide air fuel sensor is used, proportional control may be accomplished since the titanium dioxide sensor provides information on the magnitude of the air fuel ratio as well as whether the exhaust air fuel ratio is rich or lean of stoichiometry.
The prior art also teaches that a stoichiometry only sensor such as the zirconium dioxide sensor can be used to achieve a type of proportional control. That is, the zirconium dioxide sensor can be used in a way to provide some of the additional information provided by a titanium dioxide sensor. In such a method, the sensor output signal representing the air fuel ratio is modulated with a wave form with an amplitude larger than the signal selecting the air fuel ratio and the feedback is used to control the mean air fuel ratio. One such method uses two air fuel sensors, one on each or half of an engine. For example, in an eight cylinder engine there would be one sensor for each bank of four cylinders.
Due to the aforementioned time delay through the engine, care must be taken in the design of a feedback controller for use with a sensor providing proportional air fuel information. Typically, an integrator with a low gain is used as the control element. As a result, the air fuel ratio in response to a step change in air fuel ratio lasts about five engine transport delay times. If the gain of the integrator is increased to speed the system response, oscillations typically occur since the controller has not accounted for system delay time.
Referring to FIG. 2, a typical air fuel ratio feedback loop is used with a proportional exhaust gas oxygen sensor. Referring to FIG. 3, the sensed air fuel ratio of the system of FIG. 2 is shown as a function of time in response to a step input at time t=0 where the time delay is denoted as T.sub.D. For the waveform shown in FIG. 3, the gain of the system of FIG. 2 has been adjusted to prevent overshoot. When there is no overshoot, the time for the air fuel ratio to settle down to the desired level is in excess of six time delays. These are some of the problems this invention overcomes.